std::unordered_map::operator[]

From cppreference.com
T& operator[]( const Key& key );
(1) (since C++11)
T& operator[]( Key&& key );
(2) (since C++11)

Inserts a new element to the container using key as the key and default constructed mapped value and returns a reference to the newly constructed mapped value. If an element with key key already exists, no insertion is performed and a reference to its mapped value is returned.

1) Essentially performs (insert(std::make_pair(key, T())).first)->second.

2) Essentially performs (insert(std::make_pair(std::move(key), T())).first)->second.

If an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is higher than max_load_factor()*bucket_count().

Contents

[edit] Parameters

key - the key of the element to find

[edit] Return value

Reference to the mapped value of the new element if no element with key key existed. Otherwise a reference to the mapped value of the existing element is returned.

[edit] Complexity

Average case: constant, worst case: linear in size.

[edit] See also

access specified element with bounds checking
(public member function)

[edit] Example

Counts the occurrences of each word in a vector of strings.

#include <string>
#include <iostream>
#include <vector>
#include <unordered_map>
 
int main()
{
    std::vector<std::string> words = { 
        "this", "sentence", "is", "not", "a", "sentence",
	"this", "sentence", "is", "a", "hoax" 
    };
 
    std::unordered_map<std::string,size_t>  word_map;
    for (auto w : words) {
        ++word_map[w];
    }
 
    for (auto elem : word_map) {
        std::cout << elem.second
	          << " occurrences of word '"
	          << elem.first << "'\n";
    }
}

Output:

1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'